Conventional resistivity and sonic well logs now are used to determine formation pressures in Gulf Coast wells. These determinations are based on observations that log characteristics of shale vary nearly uniformly with depth through zones of normal pressure. When abnormally pressured formations are found, they usually are associated with shale that departs from the established trend.

Shale exhibits log characteristics along the normal trend when the contained fluids are free to escape at a normal rate during accumulation of overburden. However, when the formations are sealed so that further fluid loss is prevented, continued accumulation of overburden leads to overpressuring. In such formations, the shale exhibits log values typical of the depth at which the formations were sealed.

The apparent depth of formation sealing is obtained by relating the shale log values to the normal trend. For this sealing depth, a normal pressure of the fluids in the pores is computed assuming 0.465 psi. per 1 ft. of depth. Fluid pressure in the zone of interest is then computed assuming an increase at 1.0 psi. per 1 ft. of overburden accumulation subsequent to sealing. Charts simplify these computations and provide values for both the estimated pore pressure and the mud weight needed for a balance.

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